BWRRRAAAAAAAAAOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!
You are transported to a strange land.

And later:

The God Heimdall appears before you, and says:
'Well done, little human. You have vanquished the Fenris Wolf, but this is only
one battle among many fought here.'
'You are welcome to join our battle company. When you die, you will come
to Valhalla. If you wish to return here before your glorious death in battle,
blow upon my Horn again.'

Notes

The neutral zone is 5 squares wide

If you die after after finishing this quest, you'll be transported to the shrine in Valhalla instead of the one in your home city. (This has been tested and verified)

You can only kill the Fenris Wolf once. You will not be able to anger it and kill it again. Therefore, the highest number of kills for Fenris Wolf that can be achieved is 1. This means that by completing this quest, you'll be top equal on the Fenris Wolf league until the end of the month. Topping a monster kill table gives you AP bonuses.

The Fenris Wolf appears on the Bifrost Bridge. It respawns every 5 minutes. If you don't see one on the bridge, someone has killed it very recently.